1. Field of the Invention
The present invention relates to a radio communication method, a radio base station and a radio terminal used in radio communication according to IMT-2000 HSDPA or the like.
2. Description of the Related Art
Conventionally, in HSDPA (High Speed Downlink Packet Access) using IMT-2000, IMT-2000CDMA Direct Spread and IMT-2000 CDMA TDD (hereafter referred to as IMT-2000), the HARQ (Hybrid Automatic Repeat Request) system using Stop and Wait, which is simple in configuration, is used as the errored data retransmission scheme.
In this HARQ system, a down data signal is transmitted from a radio base station to a radio terminal. If the radio terminal receives the down data signal correctly, then the radio terminal transmits ACK (Acknowledgement) information to the radio base station by using an up control signal. If the radio terminal receives the down data signal erroneously, then the radio terminal transmits NACK (Not Acknowledgement) information to the radio base station by using an up control signal.
If the radio base station receives the up control signal and judges that the ACK has been received, then the radio base station transmits the next down data signal to the radio terminal. If the radio base station judges that the NACK has been received, then the radio base station retransmits the down data signal.
In the conventional HARQ system, each of the radio terminal and the radio base station has a plurality of processes 1 to N for the HARQ. Respective processes 1 to N conduct HARQ processing independently. According to this processing, lowering of throughput caused by the Stop and Wait can be reduced by preparing a plurality of processes.
It is now supposed that there is an error in a down data signal received by the radio terminal and handover occurs before retransmitted data from the radio base station is received as a result of the conventional HARQ processing. If switching to communication with another radio base station is thus conducted, then a down data signal to be retransmitted from the handover source is discarded. Therefore, the erroneous down data signal received in the beginning and stored in the radio terminal cannot be decoded either, and the erroneous down data signal is discarded. As the HARQ processes for performing communication between the radio terminal and the radio base station increase, the number of the discarded down data signals also tends to increase.
If the number of discarded down data signals increases, then throughput to other radio terminals that conduct communication with the same radio base station decreases. This results in a problem of throughput being degraded by retransmission of the discarded down data signal using an upper layer, such as the TCP/IP, in the radio terminal and the radio base station.